oxenstored: make --restart option best-effort

oxenstored: make --restart option best-effort

Only attempt to restore from saved state if it exists.

Without this, oxenstored immediately exits with an exception if the
--restart option is provided but the state file is not present.

(The time-of-check to time-of-use race isn't a concern as oxenstored is
the only thing that should write the state file.)

Signed-off-by: Jonathan Davies <jonathan.davies@citrix.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Christian Lindig <christian.lindig@citrix.com>
Release-acked-by: Julien Grall <julien.grall@arm.com>